The problem is, by the time tanks get a cinderhulk, they no longer have issues.
What world are you living in? When you get Cinderhulk on a tank you're still squishy as fuck unless you're Rammus. The item gives a block 450ish HP on completion but it really doesn't help you be tanky at that. You're still massively lacking in resists, you need 3 items to really be tanky as a tank in the jungle in most cases. Normally that's Cinder/Rands/SV.
It's the early levels that're the problem, but it's hard to buff tanks and only tanks, there.
So let them have a weakness and give them a strength in another area. If jungle tanks are weak early game they should scale up hard. Currently they don't. Someone like Lee starts stronger and keeps pace to around 3-4 items. Why would I play a tank when they take so long to scale and barely outscale anyway.
Cinderhulk is a good solution to this. It's a solution that only effects jungle tanks and doesn't help out lane tanks who aren't struggling in the same way. A simple buff like 15% increased HP>20% increased would mean an extra 100ish HP at 3 items, that's a very useful buff.
If I'm wrong and early game still winds up being a problem then my solution would be to tweak the Legendary Guardian mastery at T5 of the resolve tree to be something like 'take reduced damage from non-minion auto attacks and reflect an equal amount as magic damage'. Something like that would help tanks clear while being relatively exclusive to tanks, since it would require at least taking a resolve Keystone. With some tweaks to CotC to make it less attractive to bruisers cough %bonus HP cough you could expect most offensive junglers not to want to spec all the way down the resolve tree.